Trunk circuits



Aug. 21, 1928. 1,681,520

J. F. DAHL TRUNK CIRCUITS Filed May 31, 1927 1 Rig Patented Aug. 21,1928.

; UNITED STATES PATENT O'FFICEQ JAGK F. DAHL, OF NEWARK, NEW JERSEY,ASSIGNOR TO BELL TELEPHONE LABORA- TORIES, INCORPORATED, OF NEW YORK, N;Y., A CORPORATION OF NEW YORK.

TRUNK CIRCUITS.

Application filed ma 31, 1927; Serial No. 195,226. r

This invention relates to telephone exchange systems and moreparticularly to exchange systems in which connections are establishedfrom subscribers? lines in fullmechanical oiiices of an exchange area,serially through a plurality o1"? manual offices to wanted subscriberslines, and has for its object the improvement of the trunking facilitiesat the first manual oflice through which such connections areestablished.

In systems in which certain oiiices are equipped with mechanicalswitching apparatus, whereas other oilices employ manual operators forestablishing connections, it is found desirable that the manualoperators be given the information required for them to extend orcomplete the connections incoming from the mechanical oiiice without thenecessity of communicating orally with the calling subscriber. To thisend, each operators position in the manual office, 1s provided with callindicator mechanism, including a set of number lndicatlng devices, whichis operated automatically under the control of registering mechanism inthe mechanicaloffice, to inform the manual operator of the number andcharacter of the requiredconnection. It is customary, in arrangements ofthis character, to provide each trunk circuit incoming to an operatorsposition from the distant mechanical oflice, with a signal lamp, theillumination of which advises the operator that a call is awaitinghcrattention on the corresponding trunk; At her convenience sheassociates the calling trunk with the com mon indicating mechanism ather position, and thereupon thewanted designation is set up anddisplayed before the operator, who proceeds to complete the connection.

lVhere. no attempt is made to secure auniformity in the distribution ofthe calls to the several operators in an ofii'ceof this character, itmay frequently occur during busy hours of the day, that an excessivenumber of calls simultaneously appear onitlie trunks of any givenoperators position, while ata position adjacent thereto only acomparatively few calls are awaiting attention. In

order that the completion of calls shall not be unduly delayed on thisaccount,pr0vision is made whereby an operator adjacent to a homeoperators position may assistthe home operator by tending'to some of thecalls at-her position. i

In systems of this character, it may further be undeslrable because ofcost to extend direct trunks from eachofiice having mechanical switchingapparatus to each manual ter minatlng ofhce. In such a case, tandem offices are provided which are directly accessi ble :trom originatingmechanical oilices, at

which tandem oflices are located operators positions equipped withnumber indicating devices. These operators, in turn, trunk desiredconnections to terminating manual offices in which subscribers lines,the number of-which are indicated on the number indicating mechanism,terminate. It then becomes necessary for thetandem operator to conversewith the terminating manual operator to inform her of the desired linenumber.. It is desirable that this be done over the establishedconnection between the operators positions without the knowledge of thecalling subscriber, andthis is done by splitting the trunk at the tandemposition and connecting the tandem operators telephone circuit to thecalling end of the trunk circuit at her position.

WVhere provision-is made to enable an op erator at a position, adjacentto the home position on which a call is incoming from a mechanicaloflic'e, to attend to calls at the home position, it is necessary uponsplitting the trunk circuit to connect the adjacent operators telephonecircuit to the trunk circuit rather than the home operators telephonecircuit.

Inaccordance with this invention, means is provided for automaticallysplitting the incoming trunk circuit at one point, when it is plugged upto a trunk circuit outgoing to a terminating oflice by thetandem homeoperator, and for connecting the telephone cir can: of the home operatorto the calling end of the trunk circuit, and for splitting the trunkcircuit at another point, when it is plugged up by the tandem adjacentoperator, and for connecting the telephone circuit of the adjacentoperator to the calling end of the trunk circuit. Provision is also madewhereby the release of the incoming trunk is maintained under thecontrol of either operator after the trunk has been plugged upinto anoutgoing trunk jack.

More specifically, in accordance with one embodiment of the invention,chosen by way of example, two splitting relays l9 and 35 are provided,relay 19 being operable when the trunk circuit is plugged into anoutgoing trunk jack after the home operator has depressed her displaykey 16, and the other relay being similarly operableafter the adjacentoperator has depressed her display key Either relay splits the trunktowards its incoming end and connects its outgoing end directly totalking leads extending, in one case to the home operators telephoneset, and in the other case to talking leads extending to the adjacentoperators telephone set. Spe cific provision is also madefortransferring the holding ground of relays 17 and from the normal contactof sleeve relay 26, to ground under the control of the home operatorsrelease key 53, or a similar key at the adjacent operators position.

The invention is illustrated in the accompanying drawing, wherein acalling 'sub scribers line is shown at the left of the drawing,terminating in an o'liice X equipped with mechanical switchingapparatus, and at the right of the drawing acalled subscribers lineterminating in the manual ofliee Z, connection beingextended between theotlices X and Z through a manual tandem o'ilice Y.

To simplify the disclosure the line switch 1, district selector 2, andsender 3 of the mechanical ofl'ice X have been only diagrammaticallyillustrated. For a full disclosure of this apparatus reference may bemade to the patent to F. A. Stearn, 1,505,171, issued August 19, 1924.At the tandem ottice an interconnecting trunk circuit incoming from theoffice X has been disclosed with the com plete equipment thereof. Thecontrol and indicator circuits at the home and adjacent operatorspositions, which may be connected to all of the incoming trunk circuitsof these positions, have been illustrated by small rec tangles 18 and56. This indicator mechanism may be of the type disclosed in the patentto A. E. Lundell and E. H. Clai:'k,'1, l6 1,08 t, issued August 7, 1923.

The B operators equipment shown in outline at oflice Z is fullydisclosedin a co pending application, Serial No. 738,488, filedSeptember 18, 1921, in the name of R. S. Bailey and E. P. Bree, and acomplete disclosure thereof herein is deemed unnecessary.

It is thought that the invention may be best understood from adescription of the manner inwhich a call is extended from substation Ain otlice X to tne substation B'in otlice Z. Upon the removal of thereceiver at substa tion A, the line switch 1 functions in the usualmanner to select an idle district selector 2, and thereupon an idlesender, such as 3, becomes associated with the calling line. The callingsubscriber then' manipulates his calling dial 4 in accordance with theoffice and numerical designation of the desired line, whereupon theregisters of the sender function in the usual manner to register thefull tandem oiiice Y throu h which access to the desired terminatingoiiice Z may be obtained. The selector 2 then proceeds to hunt for andseize an idle trunk5 extending to such tandem oilice Y.

lvhen the connection is extended to the trunk 5 a circuit isestablishedforthe relay 6 ofthe trunk 5 which may be traced in part frombattery, through the right winding of relaytl, the right winding ofimpedance coil 10, the right back centactof relay 7, the upper backcontacts of relays 8 and 9,-and thence through the selector 2 and thesender 3, re-

turning through the lower back contacts of relays 8 and 9, the left backcontact of relay 7, the left winding of impedance coil 10 to groundthrough the leftwinding of relay 6. Relay '6 energizes in this circuitand establishes a circuit forrelay 12, which may be traced from battery,through the inner right bacluzontaet of relay 13, winding of relay 12.inner right back contact of relay 11, to ground 1 L1 I E at me contactoi relay 6. Lelay 12 upon ene1- gizing, closes a locking circuit foritself independent of the contact ofre'lay 13, which extends frombattery, through the inner right front contact of relay. 12, and thenceto ground as traced at the contact of relay 6. At its outer rightcontact relay 12 establishes a circuit for lit-11111 14, whereby thelamp 14 becomes lighted as a signal to the operator lUO that a call isincoming on trunk 5 at her position.

The home operator, noting the illumination of lampla, depresses thedisplay key 16 at her position, whereupon a circuit is established forrelay 17 which maybe traced from 'batteryat control circuit 18, overconductor 22, the outer leftcontact relay 21, conductor 20, the leftwinding of relay 17, the outer left front contact of relay 12, thecontact of key 16 to ground at the control circuit 18. Relay 17,operating in this circuit locks through both of its windings and itsinner right front "contact, over conductor 15 to ground at the rightnormal contact of relay 26, independently of key 16, and at its outerright contact closes an obvious circuit for relay 13. At its outer leftcontact it prepares a circuit for relay 9. Relay 13, uponenergizing,opens at its inner right contact the initial energizingcircuit of relay 12, and closes at its outer right front contact acircuit for relay 23, which may be traced from battery, through theinner right contact of relay .12 the outer right front contact of relay13,

conductor 25, to ground through the winding of relay 23. Relay 23 uponenergizing, extends conductor 25 through its inner right contact, overconductor 24, through the winding of relay 11 to battery, thus placingthe windings of relays 23 and 11 in series,but relay 11 can not ope-rateso long as battery is connected to conductor at the inner right contactof relay 12.

lVith relays 17 and 23 both energized, a'

circuit is now effective for energizing relay 9, which may be tracedfrom battery at the outer right contact of relay 23, over cond'uc tor27, winding of relay 9, conductor 28, the left contact of relay 17,conductor 36, to ground at the control circuit 18. The operation ofrelay 9 severs the incoming trunk conductors from the windings of relay6, and connects these conductors over the front contacts of relay 9 andconductors 29 and 30 to the control circuit 18, thereby causing such anoperation of the sender as to initiate the transmision of impulses tothe control circuit 18 in accordance with the registrations recorded inthe sender, to record the line designation in the control circuit 18 inthe well known manner.

The opening of the circuit of relay 6 by the operation of relay 9 causesthe release of relay 6, and in turn the release of relay 12.

Relay 12, upon releasing, removes battery at its inner right contactfrom the conductor 25 thereby permitting relay 11 to become energized inseries with relay 23'. lVith relay 12 released, the circuit of lamp 14is transferred from steady ground at the outer right front contact ofrelay 12 to interrupted ground at interrupter 34, over a circuit whichmay be traced from battery, through lamp 14, outer right back contact ofrelay 12, conductor 31, left front contact of relay 13, the conductor32, right back contact of relay 33, interrupter 34 to ground. Lamp 14then flashes as an indication to the operator that the call ins comingon trunk 5 has receivedher attention and that the desired linedesignation is being recorded on her'control circuit 18. Relay 11, uponenergizing, preparesa circuit for the splitting relay 19, and alsoprepares a circuit for establishing a bridge across the callingend ofthe trunk circuit as will hereinafter be described. p

After the complete number has become recorded in'the control circuit 18,ground is removed at the control circuit from conductor 36 and relay 9thereupon releases, disconnecting conductors 29 and 30extending to thecontrol circuit, from the'incoming end of trunk 5 and again connectingthe incoming end of the trunk to the windings of relay 6. Since thesender has now completed its func tions, it now becomes disconnectedfrom the district selector 2 and thecalling line, and the calling linebecomes connected with the incoming end of the trunk 5. When relay 6'isagain connected to the incomingend of the trunk circuit 5, itreenergizes and at its front contact and the inner right front contactof relay 11, establishes a holding circuit for relay 1'1 and shunts thewinding of relay back contact the connection 23, whereupon relay 23deenergizes, removat its outer right contact the battery connectionvtothewinding of relay 9 and also opening the initial energizing circuit ofrelay 11. V

The tandem home operator perceiving the complete number on herindicator, and noting that the oflice designation is for a lineterminating'in manual oflice Z, tests the jack 40 of a trunlcextendingto such'oflice by touching the tip of plug 39 to the sleeve of the jack40.

If the outgoing trunk is busy, she will re ce ivea busy click in hertelephone circuit, over a circuit which may be traced from plug 39 totheupper back contact of relay 41, conductor 42, outer left back contactof relay 33, conductor 43, through the tone coil of her telephonecircuit indicated diagrammatically by the rectangle 37. If the trunk isidle, as will be assumed, she inserts the plug 39 into the jack 40,thereby closing a circuit from ground through coil 44, sleeve of jack49, sleeve of plug 39, winding of relay 26 and resistance in parallel tobattery. Relay 26 operates in this circuit, closing an obvious circuitfor relay 41. At its right contacts, relay 26 transfers the lockingcircuit of relay ,17 over conductor 15, from direct ground to ground atthe normal contacts of the home operators position relay 1, the holdingcircui't now extending over the alternate right contact of relay 26, theouter right back contact of relay'52, the normal contacts of relay 64 toground.

At its uppermost lower front contact, relay 41 prepares a circuit fordisconnect lamp 338 but the lamp does not light at this time as itscircuit is open at the inner left back contact of relay 11. Relay 41opens at its upper from the tip of plug 39 to the test lead 42, and atits upper front contact extends a connection from the tablishes acircuit for the splitting relay 19,

corresponding to the home operators posi-' tion, extending from battery,resistance 58, backcontact of relay 46, in parallel with the winding ofrelay 57, thence over conductor 65, the lower intermediate normalcontact of relay 19, winding of relay 19, left back contact of relay 52,outer right front contact of relay 11, right normal contacts of relay21, to ground at the lowermost contact of relay 41. Relay 19 operates inthis circuit, looking over" its lowerintermediate alternate contactcalling subscriber can not hear the conversa tion. The home operator nowgives the B operator the number of the desired wanted attend tosuchincoming call, the ad acent line as indicated on the indicator andcontrol circuit 18.

After the tandem home operator has given the number,-she depresses thewipe out key 53,

I thereby operating relay 64, which opens the previously traced holdingcircuit of relay 17. At its alternate contact, relay 64 closes a circuitfor relay 21, which may be traced from battery, winding of relay 21,lowermost front contact of relay 19, alternate contact of relay 64 toground. Relay 21 energizes, looking over its right alternate contact toground at the lower front contact of relay 41, opening at its outer leftback contact the initial energizing circuit of relay 17, and at itsnormal right contacts opening the holding circuit of splitting relay19.1 Relay 19 thereupon deenergizes, disconnecting theoperatorstelephone circuit from conductors 48 and 49 and establishing thecontinuity of the trunk circuit between the originating ofiice X and theterminating office Z.v Relay 17 upon decnergizing opens the circuit ofrelay 13 which upon releasing opens the previously traced flashingcircuit for lamp 14, and the lamp becomes dark.

In response to the instructions given to the B operator at oflice Z, theB operatorin the we] ,known manner extends the connection over plug 68and jack 69 to the line of the desired subscriber. Upon the response ofthe called subscriber at station B, apparatus 100 functions in themanner set forth in the above mentioned specification, to reverse thedirection of current flowing through the polarized relay 47, whereuponrelay 47- energizes, closing an obvious circuit for reversing relay 7.Relay 7 nowvreverses the direction of current flowing in the trunk 5extending to the originating office X for supervisory or call chargingpurposes.

\Vhen the calling subscriber replaces his receiver on its switchhook,relay 6 releases in turn releasing relay 11. Relay 11 upon releasing,completes the circuit of the disconnect lamp 38, and at its Outer leftcontact opens the bridge through polarized relay 47. The B operator atthe terminating ofiice is thereby given a disconnect signal in the usualmanner, and proceedsto take down the: connection at ;her position. Atthe the tandeinoperator removes plug 39 from the jack 40, therebyreleasing relay 41. The release of relay 41 opens the circuit of thedisconnect lamp ,38 and opens the locking circuit of relay 19 which thenreleases. All apparatus at thetandem operators position is now restored.i 7

If at the time the call came in from the mechanical switching oflice X,the home op-, erator had been too busy with other calls to operator, iffree,would have attended to it ,by depressing her display key 54. Atthis time relays 6 and 12 are operated as pre viously described, andtherefore upon the operation of key 54 a circuit is established forrelay 55, extending from battery at the adjacent operators controlcircuit 56 over the inner left contact ofrelay 21, left wind-- ing ofrelay 55, inner left front contact of relay 12, contact of key 54 toground at the. adjacent operators control circuit 56. Relay 55, uponenergizing, closes a lockingcircuit for itself through both of itswindings and its inner right contact, the inner'right back contact of,relay 17, conductor 15 to ground at the right normal contacts of re lay26, independent of the contacts of key 54. Relay closes an obviouscircuit for relay 13, which upon energizing, causes the operation ofrelay 23 andperforms the other functions as previouslydescribed. l/Vithrelays 23 and .55 operated, a circuit is now closed, extending in seriesthrough the winding of relay Bend the right winding of relay 33, whichmaybe traced from ground at the ad acent operators control circuit 56,intermediate contact of relay 55, right winding of relay 33, conductor60, winding of relay 8, conductor 27 to battery at the outer. rightcontact of relay 23. Relay 8, upon energizing in this circuit, connectsthe incoming conductors of the trunk circuit 5 over conductors 61 and 62to the adjacent operators control circuit 56, and opens the connectionof the trunk circuit to the windings of relay 6. As before, relayGdeenergizes, releasing relay 12, whereupon relay 11 energizes;

lVith relays 11 and33energized, a circuit .is nowclosed to lock relay 33and energize relay 52, this circuit extending from battery,

through the left winding of relay 52 and the left winding ofrelay 33inparallel, through the inner left contact of relay 33 to ground at theinner left contact of relay 11. Relay 52, upon energizing, looks overits right winde ing and inner right front contact, to ground at theouter right contact of relay 55. The circuit of lamp 14 is nowtransferred from 59, over a circuit extending from battery,

through the lamp 14-, the outer right back contact of relay 12,conductor 81, left front contact'of relay "l3, conductor 32, rightfrontcontact of relay 33 to groundthrough the in terrupter 59. The lamp latnow flashes rapidly as an indication to the operators atfboth the homeand adjacent positions that the adjacent operator is attending to theincom-. ing call on trunk 5. f

\Vhen the pulses have all the adjacent operator teststhe trunlreigtendringtothe desired terminating ofiiceZ,estab-;

lishing a test circuit which" maybe traced,

from the tip of plug 89 upper back contact of relay 41, conductor42,0uterleft front con. tact of relay 33, conductor -63 to thetone coilof the adjacent operators telephone circuit,

diagrammaticallyindicated at 70;f If the? trunk tested is'idle, and theoperator-receives no click in her telephone circuit, sheinserts plug 39into jacket), In ,responselthereto relays 26a11d 41 operations before;811188,

however, relay 52 is new operated, a circuit closed for splitting relay35, extendingiirom battery at the adjacent operators telephone circuit'70, through a relayand'resistance corresponding to relay 57 andresistance 58, the intermediate lower normal contact and winding ofrelay 35, the leftfront contact of relay 52, the outer right contactofrelay 1l,normal right contacts of relay 21, to ground at the lowermostcontact of relay 41. Relay 35, upon energizing, looks over the alternatecontact of'its intermediate'lower -arrnature=,*to battery atthe adjacentoperators telephone circuit 70, through'zi relay corresponding to relay16. Relay now connects conductors l8 and 49 of the trunk circuit throughits upper and uppermostlower"front contacts to i the talking conductorsof the adjacent operators telephone'circuititolenable the operator atthe adjacent position to converse with the B operator at oilieeZ.

To wipe out the display on the indicator and control circuit 56 theadjacent operator 1 i, depresses her release key, notshown,-corresponding to'key 53 which closes a circuit for relay'21extending from battery, through the winding of relay 21, the lower frontcontact of relay 35 to ground at the alternate contact of a relay at theadjacent operators telephone circuit corresponding torelay 64; From thispoint the circuit functions in the manner previously described. f g

If the tandem operator after having CQlll pleted'a connection to the I?operaton for example in the manner previously described in: connectionwith a call handled by the tandeni hoineop erator, fails to depress thekey 53, and then proceeds with a second call over ing locked in seriesanother incoming trunk corresponding to the following I with relay 46,operating such relay 46. Vilith the relay 4-6 operated when the operatordecorresponding to key 16, and inserts the second trunk plug into thejack of an outgoing trunk, therelays 26 and all of the second e i trunkoperate,,the relay d-lYclosing a circuit been receivedand the indicatorlamps associated with control a circuit 56 displaytthe completedesignation,

for the splitting relay of the second trunk, correspondingto thewindinglS), in series withthe winding of relay 57 of the home operatorstelephone circuit. Since at thistiine relay of-the first trunk, andrelay 57 is not shunted by resistance 58, relay .57operates.

Thesplitting relay-of the second trunk beings i presseslthe displaykeyoi' the second trunk,

relay dfiisoperated in series with the splitting 'marginal'does not,however, energize in series with relay 57e vI Thesplittingmelay of thefirst trunk, hav

with and operated relay 46, a ClICllll; is now established from groundthroughthe;contact of relay :57, the inner "contact of relay 46, thelower front contactof spl tting relay 190i the first trunk to batterythrough the winding of relay 2 1. Relay 21 releases relays 19 and lT ofthe first trunk, disconnects the operators telephone ci it,

from the first trunk "and closesthistrunk i through for talking betweenthe calling and called;subscribers; WVhen relay 19 releases, relay4E6'a1so releases shunting relay 57, at its outer contact, therebycausingrelay 57:to release and'pei'niitting sullicient current to flow!through and: to operatethe splitting relay of the second trunk.corresponding to relay 19.

The operation oi'the splitting relay connects a the operators telephonecircuit to this trunk.-

lhecircuits function similarly on second calls initiated by the adjacenttandem operator. What is-claimedis: 9

1. In a telephone system, a pluralityof ope erators positions, a, trunkcircuit incoming to one of said positions and terniinating in a i plug,atrunk signal, means at, each operators position operable by theoperator thereat in response to said signal, a telephone set at eachoperatoris position, and means allotted to each operators positionresponsive to the operation of the responsive means of such position andto the insertion of said plug into an outgoing line jack fordisconnecting thetallc ing conductors of said trunk circuit andforconnecting said such operators position; r I 1- 2. In atelephone system,a plurality ofoperator's positions, a trunk cirouit incoming to oneo'fsaid positions and terniiuatingin a plug, anjindicating devicetltjSklClL position,

plugto the telephone set of means at each position "for associating. thein- 1 dicating-dev1ce of thatposition Wlth said trunk circuit,1atelephone setat each opera tors position, and means allotted'to eachoperators position responsive to the association of the indicatingdevice of such pos1t1on With said trunk circuit and tothe insertion ofsaid plug into an outgoing hne ack for disconnecting the talkingconductors of said trunk circuit and for connecting sald plug to thetelephone set oisuch operators position.

3. In a telephone system, an operators p'o- I sition, a trunk circuitincoming to said position and terminating in a plug, a trunk sig' nal,means controlled by the operator at said.

position in responseto said signal, a telephone set at said position, asecond operators position, means controlled by the operator at saidsecond position in response to said sig nal, a telephone set at saidsecond position, means allotted to the first operators positionresponsive to the operation of the responsive 'meansof that position andto the insertion of said plug nto an outgoing line 3ackfordisconnectingthe talking conductors ofsaid trunk circuit and for connecting said plugto the telephone set of that operators 'position, and means allotted tothe second opera- & tO'I SPOSltlOIlIGSpOHSlVG to the operation of theresponsivemeans ot' that-position and to the insertionof' said'plug intoan outgoing line jack for disconnecting thetalking conductors of saidtrunk circuit and for connecting said plug to the telephone set of thatoperators position. i

4. In a telephone system, an operators po' sition, a trunk ClICllltincoming'to said'position and terminating in a plug, an 1nd1cat1ngdevice at said pos1t1on, means at said position for associating saidindicating device Wltll said trunk circuit, a telephone set at I saidposition, a second operators position, an indicating device at saidsecond'position, a telephone set at'said second positioin'means atsaidsecond position for associating said second indicating device with saidtrunk,"

means allotted to the first operators position responsive to theassociation of the indicating device of that position with said trunkcircuit and to the insertion of said plug into an outgoing line jack fordisconnecting the talking conductors of said trunk circuit andforconnecting said plug to the telephone set 0t that operatofisposition,and means allotted to the second operators position responsive to theassociation of :the. indicating device of that position with said trunkcircuit and to the insertion of said plug into an outgoing line jack fordisconnecting the talking. con ductors of said trunk circuit and forconnecting said plug to the telephone set of that operator s position.t, t 5. y In a telephone system, a plurality of operators positions, atrunk circuit incoming to one of said positions and terminating in theplug, a trunk signal, meansat each operators position operable by theoperator thereat in response tosaid signal, a telephonens et at-eachoperators position, and relay means allotted to each operators positionresponsive to the operation of the responsive means of such position andto the insertion of said plug into an outgoing line jack fordisconnecting the talking conductors of said trunk circuit and forconnecting said plug to the telephone set of such operators anindicating device at said second operators 1 position, means at'saidsecondpositionfor associating said. second indicating device Withsaidtrunk,za telephone setat said second position, a relay allottedto thefirst opl of the indicating device of that position with y eratorsposition responsive to the'ass'ociation said trunk circuit and totheinsertion of said 7 I plug into an outgoing line jack for disconnectingthe talking conductors of said trunk circuit and for connecting'saidplug to the telephone set of that operators position,

and a second relay allotted to the second operators position responsiveto the association of the indicatingdevice of that position with saidtrunk circuit and to the insertion of said plug into an. outgoing linejack for disconnecting the talkingconductors of said trunk circuitandfor connecting'said plug to the telephoneset of that operators posi-;

tion.

operators" positions,- a trunk circuit incom- Ping to one ofsaidpositions and terminating 7. In a telephone system, a plurality ofin a plug,a jack, a trunk signal, means at each operatorspositionoperable by the operator thereat inresponse tosaid signal, a telephoneset at eachoperators position, a] relay in said trunk circuit allottedto each operators position responsive to the operation of the responsivemeans of such posi-; tion and to the insertion of said plug intosaidfjack for disconnecting the talking conductors of said 'trunkcircuitand for connecting said plugto vthe telephone set of such operatorsposition, a key at each posie tion, and means responsive only to the keyof theconnected position to disconnect the corn nected operators set andto reestablish the, continuity of said trunk circuit.

8. In a telephone system, an operators position, a trunk circuit'ncoming to said pos tion and terminating in a plug, a ack, a controlrelay for sa d: trunk, means controlled by the operatorto operate saidrelay, a sec- 0nd relay, a locking circuit for said first relaycontrolled bysaid second relay, a release key, and meansfor operatingsaid secondrelaytotransfer the locking circuit 8O at saidposition, a.second operators position,-:

of said first relay to the control of said release key following theinsertion of said plug into said jack.

9. In a telephone system, an operators position, a trunk circuitincoming to said 'position and terminating in a plug, a jack, anindicating device at said position, a control relay for said trunk forcausing-the association of said device with said trunk, a key for 10operating said relay, :1 second relay, a locking circuit for said firstrelay extending to ground through normal contacts of said second relay,a release key and means responsive to the insertion of said plug intosaid jack for operating said second relay to t 'ansfer said holdingcircuit to the control of said release key. I

In Witness whereof, I hereunto subscribe my name this 29 day of April,A. D. 1927.

JACK F. DAHL.

